The left mandible is overall well-preserved with only slight breakage near the incisor region, the dorsal border of the coronoid process, and the angular process. The mandibular symphysis is rough with interdigitating rugosities, which are anteroposteriorly aligned in the dorsal half and more radial on the more rugose posteroventral part. The posterodorsal corner of the symphysis is relatively smooth compared to the remaining dorsal part. The greatest anteroposterior length of the symphysis is ∼63 mm, and the greatest dorsoventral height is ∼46 mm. The long axis of the symphysis forms an angle of ∼45° with the alveolar border, measuring ∼72 mm of its length. The posterior margin of the symphysis is at the anteroposterior level of p3.

The horizontal ramus is very deep dorsoventrally, gradually becoming deeper from p1 to m3, then its ventral border rises into a small, distinct, and medially projecting marginal process, which is for the insertion of the digastric muscle. There are two mandibular foramina: the larger anterior foramen is ventral to the diastema between p2 and p3 and positioned at the dorsoventral level near the dorsal one-third of the mandibular height, and the smaller posterior foramen is ventral to the anterior border of p4 and slightly ventral to the anterior foramen. There is a rudimentary shallow premasseteric fossa anterior to the marginal process and likely extending ventral to m1 (Figure 3C). The masseteric fossa is deep and extensive for the insertion of the middle and deep layers of the masseter muscle, and its anterior part does not reach the level of m3.

The coronoid crest is inclined posteriorly and broadens in its distal half. The mandibular notch is wide and shallow, gently curved from the coronoid process to the condyloid process. The coronoid process is high and broad, overhanging the anterior border of the condyloid process. The condyloid process, positioned level with the apex of the m1 protoconid, is very robust and composed of two parts with the long axis inclined slightly medially (Figure 3B). The lateral half of the condyloid process is roughly conical, bluntly pointed laterally, and buttressed ventrally with the articular facet facing more dorsally than posteriorly. However, its medial half is semi-cylindrical with a truncated medial border that is buttressed anteriorly and with the articular facet facing posteriorly and extended to the ventral side. The articular facets of the condyloid process are convex dorsoventrally and divided by a distinct synovial fossa on the dorsal side. Although the angular process is incomplete, it is projected posteriorly and separated from the marginal process by a wide, shallow indentation. A rough, narrow triangular depression is present along the lateral side of the indentation. The margin of the mandibular foramen is partially cracked, and it is positioned slightly ventral to the condyloid process.

The incisors are not present, but three alveoli are preserved. These incisor alveoli indicate that i1-i2 are much smaller than i3, and the i1 alveolus is compressed strongly lateromedially (Figure 3A). The root of i2 is slightly smaller than that of i1, and it also is situated posteriorly and slightly labially. Therefore, i2 and i1 are aligned nearly longitudinally rather than transversely. The root of i3 is placed mostly labial to that of i1, is roughly triangular in outline, and is prominently larger than i1 and i2. The canine is large, robust, laterally compressed, and nearly erect with an oval outline in cross section, as in many arctoids. The apex of the canine is broken but was likely recurved distally.

The postcanine diastema between the canine and the most anterior alveolus is 9.73 mm in length. Posterior to the postcanine diastema, two closely placed with a 2.28 mm diastema and anteriorly inclined alveoli are interpreted as deriving from a single-rooted p1 and p2 rather than a double-rooted p2 with p1 absent (Figure 3D). Judging from the coalesced double-rooted p4 and the single-rooted p3, it is unlikely that the two alveoli belong to a double-rooted p2 with p1 absent and would contrast with the common characteristics of most carnivorans that premolars are enlarged posteriorly.

p3 has only one broken root preserved, which is separated from p2 and p4 by two diastemas with 8.29 mm and 9.84 mm length, respectively. p4 is complete and much reduced with two nearly coalesced roots (Figures 3C₁, C₂ ). The main cuspid of p4 is pointed and sharp with a flat lingual surface and a convex buccal surface. Both the anterior and posterior crests of p4 are straight and distinct, and the former is anterolingually extended, while the latter is posteriorly directed. A cingulid-like basin is present along the lingual and posterior sides of the crown with a swollen shelf on the posterior side. There is no accessory cuspid or cingulid cuspid on p4.

m1 is very large with a high trigonid. The paraconid is spear-like and composed of three facets, which are bordered by three ridges descending from the apex of the paraconid: an anterior cristid is slanted posteriorly and extends slightly lingually down to the base of the crown; a shorter lingual cristid descends to the notch between the paraconid and protoconid on the lingual side; and third, the posterior cristid forms the anterior half of the carnassial blade. The paraconid is composed of a buccal convex face and anterolingual and posterolingual flat facets. The protoconid is the highest cuspid of m1 but is smaller than the paraconid in a buccal view. The preprotocristid extends anterobuccally and forms the posterior half of the carnassial blade. A slightly worn facet is present along the buccal edge of the blade. The notch between the two cristids of the carnassial blade forms a near-right angle in the buccal view. Another blunt, indistinct ridge descends anterolingually from the protoconid to the notch between the paraconid and protoconid on the lingual side, forming a deep V-shaped notch with the lingual cristid of the paraconid. Therefore, a flat anterior face is present between the two cristids of the protoconid. The paraconid and the protoconid are separated by a distinct groove on the occlusal and lingual sides. The enamel of the lingual surface of the metaconid is partially broken. The much smaller metaconid is about half the height of the protoconid (but nearly as high as the paraconid), positioned posterolingually to the protoconid, and does not surpass the protoconid posteriorly in buccal view. The posterior surface of the trigonid is nearly vertical or slightly anteriorly slanted. The talonid is short, but not very low, and slightly narrower than the trigonid. The buccal surface of the talonid is lingually inclined, while the lingual surface of the talonid is vertical. The hypoconid crest is slightly anterolingually extended and lies in the middle of the talonid. The entoconid crest is slightly lower than the hypoconid crest, and the two crests join on the posterior side. Therefore, the talonid forms a loop on the occlusal surface. The boundary between the trigonid and talonid is demarcated by a distinct groove, which is continuous on the occlusal and buccal surfaces. There is no cingulid on m1.

The alveolus of m2 is relatively large and composed of two equal-sized portions, indicating m2 is double-rooted and not reduced. The alveolus of m2 is oriented slightly obliquely rather than perpendicularly to the long axis of the crown. The alveolus of m3 is much smaller than that of m2 and oval in outline, indicating m3 is single rooted. Furthermore, the size of the molars becomes smaller posteriorly from m1 to m3. The long axis of the molar series is slightly anterobuccally extended, whereas that of the premolar series is slightly anterolingually directed. Therefore, the angle between the two axes is about 160°.

The most conspicuous characteristics of the late Eocene Lonchocyon are the spaced, highly reduced premolars with single-rooted p1-3, and the two fused roots of p4. Among the five subfamilies of Amphicyonidae (i.e., Haplocyoninae, Temnocyoninae, Daphoeninae, Thaumastocyoninae, and Amphicyoninae), Haplocyoninae and Temnocyoninae share a synapomorphy of developed premolars as sister groups (Hunt, 2011), and the North American endemic Daphoeninae has unreduced premolars. By contrast, both Thaumastocyoninae and Amphicyoninae tend to reduce the premolars, but the earliest members of these two subfamilies from the Paleogene still retain the primitive unreduced premolars, unlike the new specimen. For instance, the earliest thaumastocyonine Ysengrinia tolosana from MP 30 and the amphicyonine Cynodictis from the late Eocene have a well-developed p4 with a posterior accessory cuspid and a p3 with two roots, while Cynodictis has a much smaller size than Y. tolosana and the new specimen (Kuss, 1965; Bonis, 1978; Heizmann and Kordikova, 2000; Solé et al., 2021). The amphicyonine Pseudocynopsis and Cynelos from Quercy, France, have reduced, spaced premolars, but p2-4 retains two separate roots (Ginsburg, 1965; 1966; Kuss, 1965). The presence of a single root of p3 has been reported in the Late Miocene amphicyonine Magericyon. However, the latter lacks dp1/p1-p2 and has a double-rooted p4 (Peigné et al., 2008). Ursidae also exhibits a trend toward reduced premolars. However, only Pliocene and extant Ursus could have a single root in p1-3, which is even sometimes absent. Hemicyoninae and early members of Ursinae Ballusia and Ursavus have reduced, simple premolars, but p2-3 is double-rooted and p4 is relatively large (Qiu et al., 1985; Ginsburg and Morales, 1998; Qiu et al., 2014).

The p4 of Lonchocyon is characterized by its rather small size with two fused roots and a small posterior shelf, as well as the lack of a posterior accessory cuspid. The length ratio of p4 to m1 is 0.31 in Lonchocyon (Figure 4A), which is less than the ratio in Magericyon (0.38, 0.42), while the ratios in other genera of amphicyonines (0.44–0.67) and thaumastocyoninae (0.48–0.71) are usually much greater (Solé et al., 2022). The ratio of p4 to m1 length in the ursine Ballusia and Ursavus ranges from 0.44 to 0.61 (Qiu et al., 2014), and the ratio in Ursus minimus ranges from 0.45 to 0.59 (Baryshnikov and Lavrov, 2013). The p4 of Lonchocyon has no posterior accessory cuspid, which is in contrast to the Amphicynodontidae, hemicyonine Cephalogalini, Phoberocyon, and most Amphicyonidae with a distinct posterior accessory cuspid on p4 (Hunt, 1998b; Ginsburg and Morales, 1998). However, the posterior accessory cuspid on p4 also is reduced or even absent in some amphicyonids, such as Guangxicyon (Zhai et al., 2003), the amphicyonine Magericyon (Peigné et al., 2008), and the Pseudarctini, which comprises Pseudarctos, Ictiocyon, and Dehmicyon (Morales et al., 2021b). p4 has lost the posterior accessory cuspid in some derived Ursidae, including Hemicyon, Zaragocyon, Plithocyon, and Ursinae (Ginsburg and Morales, 1995; Ginsburg and Morales, 1998; Hunt, 1998b).

The m1 of Lonchocyon is characterized by its larger size (length = 27.28 mm) compared to contemporaneous Amphicyonidae and Hemicyoninae and by its large, spear-like paraconid (Figure 4B). The m1 paraconid of Lonchocyon is composed of three faces bordered by a slanted, anterior cristid, a lingual cristid, and a posterior cristid. The lingual cristid is present in some mustelids like extant Gulo gulo. By contrast, the lingual cristid is absent and the lingual surface of the paraconid is somewhat swollen in the compared Paleogene Amphicyonidae and hemicyonine Cephalogalini (Hunt, 1998a; Bonis, 2013). In addition, a faint, blunt ridge descends from the anterolingual surface of the protoconid on m1 of Lonchocyon, forming a triangular anterior surface of the protoconid with the preprotocristid. A similar anterior surface of the protoconid on m1 seems only present in the Oligocene Cephalogalini Adelpharctos, according to Bonis (1971). The metaconid of m1 is moderately reduced, nearly as high as the paraconid, and not retracted, relative to the protoconid in Lonchocyon, which is similar to the development of the m1 metaconid in early amphicyonine Cynodictis palmidens (Bonis, 1978), Pseudocyonopsis (Kuss, 1965), haplocyonine Haplocyon dombroskyi (Bonis, 1966), and thaumastocyonine Y. tolosana (Kuss, 1965). By contrast, the m1 metaconid is either relatively large in the daphoenine Daphoenus, Brachyrhynchocyon (Loomis, 1931; Scott and Jepsen, 1936), and haplocyonine Parhaplocyon (Bonis, 1966) or more reduced or even absent in the daphoenine Daphoenictis (Hunt, 1974), derived temnocyonines (Hunt, 2011), haplocyonine Haplocyon crucians, Haplocyonides (Hürzeler, 1940; Bonis, 1966), some derived amphicyonines like Magericyon (Peigné et al., 2008), and derived thaumastocyoninae (Morales et al., 2019; Morales et al., 2021a). The hemicyonine ursids are characterized usually by a retracted metaconid relative to the protoconid on m1, which is different from Amphicyonidae, Amphicynodontidae, and Lonchocyon. However, the m1 metaconid is not retracted in the hemicyonine cephalogalini Filholictis (Cirot and Bonis, 1992; Bonis, 2013). On the other hand, the m1 metaconid is slightly retracted in the haplocyonine Parhaplocyon (Bonis, 1966) and strongly retracted (but sometimes lost) in Gobicyon (Jiangzuo et al., 2019). The talonid of m1 in Lonchocyon is more similar to early ursids than to amphicyonids in being nearly as wide as the trigonid and forming a shallow basin with the almost equally developed hypoconid crest and entoconid crest joining posteriorly. By contrast, the m1 talonid is usually slightly narrower than the trigonid (sometimes wider; for instance, in Crassidia (Heizmann and Kordikova, 2000)) hypoconid (crest) dominates the talonid with a low entoconid (crest), and the talonid opens posteriorly in amphicyonids (Kuss, 1965; Hunt, 1998a; 2011; Morales et al., 2021b). However, the hemicyonine Cephalogalini differs from Lonchocyon by having a continuous lingual ridge of the talonid gently joining the metaconid without any notch and displaying a relatively wider basin of the talonid with a more buccally placed hypoconid crest (Bonis, 2013).

The ratio of m2 length to m1 length is 0.63 in Lonchocyon (Figure 4C), similar to the ratios in early thaumastocyonine Ysengrini (Crassidia and Ysengrinia) (0.58–0.62) but greater than the more derived Thaumastocyonini (0.37–0.54) (Solé et al., 2022). Furthermore, the talonid of m2 is shorter and narrower than the trigonid in the earliest thaumastocyonine Y. tolosana from the late Oligocene, but the talonid alveolus is slightly wider than the trigonid alveolus in Lonchocyon. Among the three tribes in Amphicyoninae, the ratio of m2 length to m1 length in Lonchocyon is close to those of Amphicyonini (0.63-0.71), Pseudocyon (0.6, 0.64), and Pseudarctini Dehmicyon schlosseria (0.59), greater than that of the Magericyonini Magericyon (0.45, 0.54) and lesser than that of the Pseudarctini Ictiocyon (0.72) and Pseudarctos (0.71) (Morales et al., 2021b; Solé et al., 2022). The early amphicyonine Pseudocyonopsis from the Oligocene to early Miocene has a similar ratio between m2 length to m1 length (0.59, 0.61) to Lonchocyon (Kuss, 1965). Among the Hemicyoninae, the ratio of m2 to m1 length in the Cephalogalini is the variable between 0.51 and 0.71 (Bonis, 2013), and the ratios of the early Hemicyon, H. gargan, and Zaragocyon (0.58–0.61) are similar to that of Lonchocyon (Ginsburg and Morales, 1995; Ginsburg and Morales, 1998).

The lower jaw of Lonchocyon is very large, with a deep horizontal ramus and a distinct marginal process, which are distinguished from relatively slender mandibles without marginal processes in early Amphicyonidae and Hemicyoninae (Scott and Jepsen, 1936; Ginsburg, 1966; Bonis, 2013). Few caniform taxa have a marginal process, except for Ursus, Ailuropoda (Davis, 1964), and some fossil ursids like Ursavus tedfordi (Qiu et al., 2009). The marginal process is the main insertion site for the digastric muscle in Ursus and Ailuropoda (Davis, 1964). This feature may not have many phylogenetic implications with the currently limited sample, but it does suggest that Lonchocyon likely had powerful digastric muscles, as in Ursus and Ailuropoda. In addition, the mandible of Lonchocyon has a shallow depression along the posterior ventral border of the horizontal ramus on the lateral surface, possibly suggesting a rudimentary premasseteric fossa (Figure 3C), which is more distinct in derived hemicyonines, some derived amphicyonids like Gobicyon and Ammitocyon, and some ursines (Frick, 1926; Hunt, 1998b; Jiangzuo et al., 2019; Morales et al., 2021a).

There are only two unequivocal amphicyonids with supporting detailed descriptions from the Eocene of Asia. One is a small indeterminate amphicyonid with a right M2 (for the second upper molar, rather than lower molar) (length: 6.35 mm; width: 8.5 mm) found in the Upper Eocene Ergilin Dzo Formation of Mongolia (Egi et al., 2009). Few comparisons can be made with our new specimen because of the lack of lower dentitions. However, the amphicyonid from the Ergilin Dzo Formation is much smaller than the specimen IVPP V 28616. Another is Guangxicyon sinoamericanus, an aberrant, short-jawed amphicyonid from the middle Eocene Nadu Formation of the Bose Basin, Guangxi Province, southern China (Zhai et al., 2003; Wang et al., 2019 for timescale). Lonchocyon differs from Guangxicyon in having a larger size (Guangxicyon m1 length and width are 20.6 mm and 9.2 mm, respectively), a deeper mandible, a single-rooted p3, a much-reduced p4, an m1 with more trenchant trigonid and a shallow-basined talonid, and a double-rooted m2. However, Guangxicyon is also very aberrant among amphicyonids in terms of its early development of brachygnathy with a single-rooted p2 and m2 instead of losing p1-3 or m2-3.

Only a few Cephalogalini have been discovered, with scarce materials in Paleogene Asia. A left M2 (the second upper molar) of Cephalogale sp. (IVPP V 12429) from the early Oligocene of Saint Jacques (Wang and Qiu, 2003) and a left ramal fragment with m2 of ?Cephalogale sp. (MAE SG.97.5396) from the early Oligocene Hsanda Gol Formation (Wang et al., 2005) are the only reported Asian Paleogene Cephalogalini. Our specimen can make few comparisons with them due to the lack of comparable materials, although the former is evidently larger than these Cephalogale.

In summary, Lonchocyon is unique due to its relatively large size, highly reduced premolars separated by diastemas, and spear-like paraconid on m1 compared to other Paleogene arctoids. The carnassial tooth and lower jaw show a combination of both Amphicyonidae and Hemicyoninae traits, with the trigonid having a reduced metaconid that is not retracted, which is similar to the former and having a shallowly basined talonid of m1 and a rudimentary premasseteric fossa, which are probably allied with both Amphicyonidae and Hemicyoninae. There is no doubt that Lonchocyon represents an early offshoot of Arctoidea, but its phylogenetic relationships among amphicyonids or early ursids remain unclarified since the discovery of additional complete material is pending.